1. Field of the Invention
The present invention relates to a method for preparing a so-called yttrium 66 boride (hereinafter referred to as "YB66") crystal which is useful for soft X-ray monochromator.
2. Discussion of Background
YB66 has a large lattice constant (23.4 .ANG.) and excellent properties as a monochromator crystal which can monochromatize a soft X-ray in energy region from 1,070 electron volt to 3,000 electron volt by using a (400) reflection with a lattice d spacing of 5.86 .ANG. without any intrinsic absorption. Further, YB66 is hard and has high mechanical strength, and it is hardly destroyed by irradiation by strong synchrotron beam. Thus, YB66 is useful as a soft X-ray monochromator crystal for synchrotron radiation.
A floating zone method has been known as a method for preparing a YB66 crystal, wherein a raw polycrystalline rod having an atomic composition ratio of B/Y=62 which is a congruent melting composition as indicated by point R in the YB66 phase diagram in FIG. 1, is melted to form a molten zone. Then, the molten zone having the atomic ratio of B/Y=62 is zone-passed, whereby a crystal having the atomic ratio of B/Y=62 is grown.
However, in this floating zone growth, since the molten zone is formed by melting the raw polycrystalline rod having the atomic composition ratio of B/Y=62 which is the congruent melting composition, the growth is carried out under the condition that the composition of the polycrystalline rod is coincident with those of the molten zone and the growing crystal. For this reason, the growth of the crystal is carried out at the highest melting point by solidifying the melt, and thus it is not easy to obtain a high quality crystal.